Thermodynamic evaluation of abandoned-tunnel-based compressed air energy storage under variable- and constant-pressure operation

The increasing share of renewable energy has intensified the need for efficient large-scale energy storage systems. Compressed air energy storage (CAES) is a promising option, but securing large storage space remains a major challenge. This study proposes a 1.5 MW/3 MWh CAES system using abandoned tunnels and presents a thermodynamic comparison between variable-pressure (VP) and constant-pressure (CP) operation modes. An air-bag-based concept is introduced for CP operation, in which the air bag dynamically changes its volume to maintain stable storage pressure while separating compressed air from tunnel water. Leakage tests of a thermoplastic polyurethane (TPU) air bag at 10 bar yielded a measured permeability of 1750 cm 3 ·mm/m 2 ·day·atm and a leakage rate of approximately 0.28% per hour, providing preliminary experimental support for its use as an air–water separation barrier. Three CP-CAES configurations with water storage volumes of 1293, 2000, and 3000 m 3 were compared with an equivalent VP-CAES system, revealing a clear trade-off between tunnel construction volume and the pump energy required for pressure regulation. At a representative thermal energy storage (TES) discharge temperature of 523.15 K, the RTE of CP-CAES increased from 59.55% to 63.63% as the water storage volume increased, compared with 61.84% for the VP-CAES. When realistic operating discharge temperatures were considered, the effective TES discharge temperatures were estimated to be 519.5 K for the VP-CAES and 533 K for the CP-CAES. Under these conditions, the RTE advantage of CP-CAES over VP-CAES increased with water storage volume, reaching up to 3.50%p. These results indicate that abandoned-tunnel-based CP-CAES can outperform VP-CAES when the water storage volume is sufficiently large to reduce the pump-work penalty.